Great news! (thanks Ray)
The adaptive algorithm appears to work very quickly (the problem seems to have been caused by a nearby Sure Signal box 0.8m away). A short period of time (two days) later, looking at the stats this evening:
The interleaving has dropped way back and ping speeds are at 15 ms;
The attainable speed has increased slightly from 86960 to 87416 kbps; and
The actual speed went from 70291 to 79999.
I wonder if this means a metal housed ADSL modem is a good precaution for those (most) of us with nearby SureSignal boxes?